A variety of growth factors relating to the blood system are under investigation, and erythropoietin (EPO) as an erythropoiesis stimulating agent and granulocyte colony-stimulating factor (G-CSF) as a leukopoiesis stimulating agent, among others, have been clinically put to use to this day. Regarding the technology for proliferating of hematopoietic stem cells, various cytokines such as stem cell factor (SCF), macrophage colony-stimulating factor (M-CSF), etc. were explored but none has been found to be effective enough in causing hematopoietic stem cells to multiply sufficiently in the undifferentiated form.
As the result of an intensive investigation, the inventors of this invention found that hematopoietic stem cells can be successfully caused to multiply in the undifferentiated state well by using insulin-like growth factor I (IGF-I) in combination with at least one protein selected from among SCF, M-CSF, and G-CSF. This invention has been developed on the basis of the above finding.